Martensitic Stainless Steel

ABSTRACT

Martensitic stainless steel according to the invention contains, by mass, 0.001% to 0.01% C, at most 0.5% Si, 0.1% to 3.0% Mn, at most 0.04% P, at most 0.01% S, 10% to 15% Cr, 4% to 8% Ni, 2.8% to 5.0% Mo, 0.001% to 0.10% Al, at most 0.07% N, 0% to 0.25% Ti, 0% to 0.25% V, 0% to 0.25% Nb, 0% to 0.25% Zr, 0% to 1.0% Cu, 0% to 0.005% Ca, 0% to 0.005% Mg, 0% to 0.005% La, and 0% to 0.005% Ce, with the balance being Fe and impurities, and the steel satisfies Expressions (1) and (2) and has a yield stress in the range from 758 MPa to 860 MPa. In this way, in the martensitic stainless steel according to the invention, the tempering temperature range at which a yield stress in the range from 758 MPa to 860 MPa can be obtained is increased. 
       922.6−554.5C−50.9Mn+2944.8P+1.056Cr−81.1Ni+95.8Mo−125.1Ti−1584.9Al−376.1N≧600   (1) 
       30C+0.5Mn+Ni+0.5Cu−1.5Si−Cr−Mo+7.9≧0   (2)

TECHNICAL FIELD

The present invention relates to martensitic stainless steel, and morespecifically to martensitic stainless steel for use in a corrosiveenvironment including corrosive substances such as hydrogen sulfide,carbon dioxide gas, and chloride ions.

BACKGROUND ART

As deeper oil wells and gas wells have come to be dug, there has been ademand for stronger and tougher martensitic stainless steel used as asteel material for oil well such as oil country tubular goods.Martensitic stainless steel having a yield stress (0.2% proof stress) inthe range from 758 MPa to 860 MPa (hereinafter referred to as “110 ksigrade”) and martensitic stainless steel having a strength equal to orhigher than the 110 ksi grade have been developed.

Martensitic stainless steel for oil well must have high corrosionresistance such as SCC (Stress Corrosion Cracking) resistance and SSC(Sulfide Stress Cracking) resistance. This is because oil wells and gaswells exist in corrosive environments that include corrosive substancessuch as hydrogen sulfide, carbon dioxide gas, and chloride ions. Morespecifically, martensitic stainless steel for use in oil wells must havehigh strength, high toughness, and high corrosion resistance.

Martensitic stainless steel having high strength and high corrosionresistance is disclosed by JP 2003-3243 A. The disclosed martensiticstainless steel contains at least 1.5% by mass of Mo and allows higherSSC resistance than conventional martensitic stainless steel to beobtained.

If the Mo content is high, the tempering temperature range that allowsthe 110 ksi grade strength to be obtained (hereinafter referred to as“tempering temperature range”) is very small. FIG. 1 shows the relationbetween the yield stress of martensitic stainless steel with a high Mocontent (hereinafter referred to as “high Mo martensitic stainlesssteel) and the tempering temperature. The high Mo martensitic stainlesssteel in FIG. 1 contains, by mass, 0.016% C, 11.8% Cr, 7.2% Ni, and 2.9%Mo, with the balance being Fe and impurities. With reference to FIG. 1,the gradient of the tempering temperature curve C10 in the yield stressrange from 758 MPa to 860 MPa is large. Therefore, the temperingtemperature must be about in the range from 580° C. to 600° C. in orderto obtain the 110 ksi grade strength for the high Mo martensiticstainless steel. More specifically, the tempering temperature range ΔTthat allows the 110 ksi grade strength to be obtained is very small.

If the tempering temperature range ΔT is small, the productivity isreduced. In general, several hundred tons of such high Mo martensiticstainless steel is successively produced. In this case, the high Momartensitic stainless steel is made by a plurality of heats (moltensteel produced by a single steel making process) and the chemicalcompositions of the heats are not completely the same and slightly varyamong them. If the tempering temperature range ΔT is small, thetempering temperature must be changed every time the chemicalcomposition changes in order to obtain the 110 ksi grade strength forthe steel. In short, in order to obtain the 110 ksi grade strength, thetempering temperature must be changed for each of the heats. Thenecessity of changing the tempering temperature setting in this mannerlowers the productivity.

Note that International Publication No. WO 2004/57050 is a patentdocument related to the invention.

DISCLOSURE OF THE INVENTION

It is an object of the invention to provide martensitic stainless steelhaving a large tempering temperature range that allows a yield stress inthe range from 758 MPa to 860 MPa to be obtained.

The inventor conducted various experiments and examinations and hasobtained the following findings.

(A) If the martensitic stainless steel has such a chemical compositionthat the transformation point A_(c1) of the steel is high, the temperingtemperature range that allows the yield stress to be from 758 MPa to 860MPa increases. If the transformation point A_(c1) is lower, austeniteforms during high temperature tempering process, which reduces thestrength.

(B) In addition to the increase in the transformation point A_(c1), theC content is reduced. In this way, the tempering temperature range thatallows the yield stress to be from 758 MPa to 860 MPa even moreincreases. This is because as the C content is higher, the gradient ofthe tempering temperature curve in the yield stress range of 758 MPa to860 MPa increases.

(C) If the C content is lowered, δ ferrite is more likely to begenerated, which affects the strength and toughness of the steel. The110 ksi grade martensitic stainless steel for use in environments wherethe atmospheric temperature is less than 0° C. must have both highstrength and high toughness. If the steel has a composition that allowsthe structure of the steel to become martensitic even though thetransformation point A_(c1) is raised and the C content is lowered, δferrite can be prevented from forming and the toughness can be preventedfrom being lowered while the 110 ksi grade strength is maintained.

As a result of consideration based on these findings, it was found thatif the C content is 0.01% or less, and the following Expressions (1) and(2) are satisfied, the tempering temperature range that allows the yieldstress to be from 758 MPa to 860 MPa can be larger than conventionalcases.

922.6−554.5C−50.9Mn+2944.8P+1.056Cr−81.1Ni+95.8Mo−125.1Ti−1584.9Al−376.1N≧600  (1)

30C+0.5Mn+Ni+0.5Cu−1.5Si−Cr−Mo+7.9≧0  (2)

where the contents of the elements in percentage by mass are substitutedfor the characters representing the elements.

The left side of Expression (1) (hereinafter the left side of Expression(1)=F1) is an expression used to estimate the A_(c1) transformationpoint of the martensitic stainless steel according to the invention. Asdescribed above, if the A_(c1) transformation point is high, retainedaustenite can be prevented from being precipitated during temperingprocess, so that the yield stress can be prevented from being abruptlylowered. Stated differently, the gradient of the tempering temperaturecurve in the yield stress range from 758 MPa to 860 MPa can be small.

Note that F1≧600 because the tempering process is carried out at 600° C.or less. If the tempering temperature is set to 600° C. or more,microscope carbide or intermetallic compounds in the steel becomecoarse, and this rather reduces the strength and toughness. Since thetempering temperature is 600° C. or less, it is only necessary that F1be 600° C. or more. Expression (2) is an expression used to make thesteel after tempering martensitic. If the contents of C, Mn, and Ni thatare austenite forming elements and the contents of Si, Cr, and Mo thatare ferrite forming elements satisfy the relation defined by Expression(2), the structure becomes martensitic, and δ ferrite can be preventedfrom being produced. Therefore, the strength can be prevented from beinglowered, and high toughness can be maintained.

Note that if the martensitic stainless steel does not contain optionalelements Ti and Cu, the “Ti” and “Cu” in Expressions (1) and (2) areboth “0.”

If these expressions are satisfied, a tempering temperature curve ascurve C1 shown in FIG. 2 can be obtained, and the gradient of thetempering temperature curve in the yield stress range from 758 MPa to860 MPa can be smaller than that in the conventional cases. Therefore,the tempering temperature range ΔT1 that allows the yield stress to bein the range from 758 MPa to 860 MPa is larger than the temperingtemperature range ΔT2 of the conventional tempering temperature curveC2. Therefore, the decrease in the productivity because of temperaturesetting changes during operation can be prevented.

The inventor completed the following invention based on theabove-described findings.

Martensitic stainless steel according to the invention contains, bymass, 0.001% to 0.01% C, at most 0.5% Si, 0.1% to 3.0% Mn, at most 0.04%P, at most 0.01% S, 10% to 15% Cr, 4% to 8% Ni, 2.8% to 5.0% Mo, 0.001%to 0.10% Al, at most 0.07% N, 0% to 0.25% Ti, 0% to 0.25% V, 0% to 0.25%Nb, 0% to 0.25% Zr, 0% to 1.0% Cu, 0% to 0.005% Ca, 0% to 0.005% Mg, 0%to 0.005% La, and 0% to 0.005% Ce, with the balance being Fe andimpurities, the steel satisfies Expressions (1) and (2) and has a yieldstress in the range from 758 MPa to 860 MPa.

922.6−554.5C−50.9Mn+2944.8P+1.056Cr−81.1Ni+95.8Mo−125.1Ti−1584.9Al−376.1N≧600  (1)

30C+0.5Mn+Ni+0.5Cu−1.5Si−Cr−Mo+7.9≧0  (2)

where the contents of the elements in percentage by mass are substitutedfor the characters representing the elements.

If the optional elements Ti and Cu are not contained, the “Ti” and “Cu”in Expressions (1) and (2) are both “0.” The 0.2% proof stresscorresponds to the yield stress.

In the martensitic stainless steel according to the invention, thegradient of the tempering temperature curve can be reduced by settingthe C content to 0.01% or less. In addition, the A_(C1) transformationpoint can be higher than the conventional examples if Expression (1) issatisfied. Therefore, the gradient of the tempering temperature curve isreduced, and the tempering temperature range that allows the yieldstress to be in the range from 758 MPa to 860 MPa is increased ascompared to the conventional examples.

The strength can be prevented from being less than 110 ksi as Expression(2) is satisfied, and the high toughness can be maintained. Since the Mocontent is high, high corrosion resistance is obtained.

The martensitic stainless steel according to the invention preferablycontains at least one of 0.005% to 0.25% Ti, 0.005% to 0.25% V, 0.005%to 0.25% Nb, and 0.005% to 0.25% Zr.

The martensitic stainless steel according to the invention preferablycontains 0.05% to 1.0% Cu.

The martensitic stainless steel according to the invention preferablycontains at least one of 0.0002% to 0.005% Ca, 0.0002% to 0.005% Mg,0.0002% to 0.005% La, and 0.0002% to 0.005% Ce.

In this way, the hot workability of the martensitic stainless steel isimproved. Note that these elements if contained do not affect theeffects of the invention described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the relation between the yield stress of a conventionalhigh Mo martensitic stainless steel and the tempering temperature; and

FIG. 2 shows the relation between the yield stresses of sample materials1 and 14 according to an embodiment of the invention and the temperingtemperature.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, an embodiment of the invention will be described in detail inconjunction with the accompanying drawings, in which the same orcorresponding portions are denoted by the same reference characters, andtheir description will not be repeated.

1. Chemical Composition

Martensitic stainless steel according to an embodiment of the inventionhas the following composition. Hereinafter, “%” related to each elementrefers to “% by mass.”

C, 0.001% to 0.01%

If the C content is excessive, the gradient of the tempering temperaturecurve becomes steep, and steel having a yield stress in the range from758 MPa to 860 MPa cannot stably be produced. The C content should belimited to a small value. If the C content is less than 0.001%, on theother hand, the manufacturing cost increases. Therefore, the C contentis in the range from 0.001% to 0.01%, preferably from 0.001% to 0.008%.

Si: 0.5% or less

Silicon is effectively applied as a deoxidizing agent. On the otherhand, Si hardens steel and therefore an excessive Si content degradesthe toughness and workability of the steel. Silicon is a ferrite formingelement and therefore prevents the steel from becoming martensitic.Therefore, the Si content is not more than 0.5%, preferably 0.3% orless.

Mn: 0.1% to 3.0%

Manganese contributes to improvement in the hot workability of thesteel. Furthermore, Mn is an austenite forming element and contributesto formation of a martensitic structure. However, an excessive Mncontent degrades the toughness. Therefore, the Mn content is in therange from 0.1% to 3.0%, preferably from 0.3% to 1.0%.

P: 0.04% or less

Phosphorus is an impurity and causes SSC to be generated, and thereforethe P content is limited as much as possible. The P content is 0.04% orless.

S: 0.01% or less

Sulfur is an impurity and lowers the hot workability. Therefore, the Scontent is limited as much as possible. The S content is 0.01% or less.

Cr: 10% to 15%

Chromium contributes to improvement in corrosion resistance in a wetcarbon dioxide gas environment. On the other hand, Cr is a ferriteforming element and an excessive Cr content makes it difficult to formtempered martensite, which lowers the strength and the toughness.Therefore, the Cr content is in the range from 10% to 15%, preferablyfrom 11% to 14%.

Ni: 4% to 8%

Nickel is an austenite forming element and necessary for the structureafter tempering to become martensitic. If the Ni content is too small,the structure after the tempering contains much ferrite. On the otherhand, an excessive Ni content causes the structure after the temperingto have much austenite. Therefore, the Ni content is in the range from4% to 8%, preferably from 4% to 7%.

Mo: 2.8% to 5.0%

Molybdenum is a critical element that contributes to improvement in SSCresistance and strength. In the martensitic stainless steel according tothe embodiment, the lower limit for the Mo content to allow high SSCresistance to be obtained is 2.8%. Molybdenum is a ferrite formingelement and excessive addition of the element prevents the structurefrom becoming martensitic. The upper limit for the Mo content istherefore 5.0%. The Mo content is preferably in the range from 2.8% to4.0%.

Al: 0.001% to 0.10%

Aluminum is effectively applicable as a deoxidizing agent. On the otherhand, an excessive Al content causes many inclusions to be generated,and the corrosion resistance is lowered. Therefore, the Al content isfrom 0.001% to 0.10%, preferably from 0.001% to 0.06%.

N: 0.07% or less

Nitrogen forms a nitride and lowers the corrosion resistance. Therefore,the N content is 0.07% or less.

Note that the balance consists of Fe and impurities. The impurities aremixed in the manufacturing process for various reasons.

The martensitic stainless steel according to the embodiment furthercontains at least one of Ti, V, Nb, and Zr if required.

Ti: 0% to 0.25%

V: 0% to 0.25%

Nb: 0% to 0.25%

Zr: 0% to 0.25%

Note that Ti, V, Nb, and Zr are optional elements. These elements fix Cand reduce variations in strength. On the other hand, an excessivecontent of any of these elements prevents the structure after temperingfrom becoming martensitic. Therefore, the content of each of theseelements is set to the range from 0% to 0.25%, preferably from 0.005% to0.25%, more preferably from 0.005% to 0.20%.

The martensitic stainless steel according to the embodiment contains Cuif required.

Cu: 0% to 1.0%

Copper is an optional element and an austenite forming element as withNi suitable for making the structure after tempering martensitic. On theother hand, an excessive Cu content lowers the hot workability.Therefore, the Cu content is from 0% to 1.0%, preferably from 0.05% to1.0%.

The martensitic stainless steel according to the embodiment furthercontains at least one of Ca, Mg, La, and Ce if required.

Ca: 0% to 0.005%

Mg: 0% to 0.005%

La: 0% to 0.005%

Ce: 0% to 0.005%

Note that Ca, Mg, La, and Ce are optional elements. These optionalelements contribute to improvement in hot workability. On the otherhand, excessive contents of these elements cause coarse oxides to begenerated, which lowers the corrosion resistance. Therefore, thecontents of these elements are all in the range from 0% to 0.005%,preferably from 0.0002% to 0.005%. Among these elements, Ca and La areelements that particularly contribute to improvement in hot workability.

2. Manufacturing Method

Steel having the above-described chemical composition is melted andrefined by well-known refining process. Then, the molten steel is formedinto a continues casting material by a continues casting method. Thecontinuos casting material is for example a slab, bloom, or billet.Alternatively, the molten steel may be made into ingots by an ingotcasting method.

The slab, bloom, or ingot is formed into billets by hot working. At thetime, the billets may be formed by hot rolling or by hot forging.

The billets produced by the continues casting or hot working aresubjected to further hot working and formed into oil country tubulargoods. For example, Mannesmann process may be performed as the hotworking. Alternatively, Ugine-Sejournet hot extrusion process may beemployed as the hot working, while a forged pipe making method such asEhrhardt method may be employed. The oil country tubular good after thehot working is subjected to quenching process and tempering process. Thequenching process is carried out according to a well-known method. Thequenching temperature is for example from 900° C. to 950° C., whileother temperature ranges may be employed.

In the tempering process, the lower limit for the tempering temperatureis preferably 500° C. If the tempering temperature is too high, retainedaustenite is precipitated, so that the yield stress cannot be in therange from 758 MPa to 860 MPa. Therefore, the upper limit for thetempering temperature is preferably 600° C.

The martensitic stainless steel according to the embodiment of theinvention satisfies the following Expressions (1) and (2):

922.6−554.5C−50.9Mn+2944.8P+1.056Cr−81.1Ni+95.8Mo−125.1Ti−1584.9Al−376.1N≧600  (1)

30C+0.5Mn+Ni+0.5Cu−1.5Si−Cr−Mo+7.9≧0  (2)

If Expression (1) is satisfied, the A_(c1) transformation point is high,and therefore the gradient of the tempering curve in the yield stressrange from 758 MPa to 860 MPa can be reduced. If Expression (2) issatisfied, the structure can become martensitic in an acceleratedmanner. Therefore, if both Expressions (1) and (2) are satisfied, thetempering temperature range that allows the yield stress to be in therange from 758 MPa to 860 MPa can be larger than those of theconventional examples. Therefore, a decrease in productivity based onchanges in the temperature setting during operation can be reduced.

High toughness necessary for steel for use in an oil well can beobtained by satisfying Expression (2).

Note that if the martensitic stainless steel does not contain Ti and Cuas optional elements, “Ti” and “Cu” in Expressions (1) and (2) are“zero.”

In the above-described example, the martensitic stainless steel is madeinto a steel pipe, but the steel may be formed into a steel plate.

EXAMPLE 1

Sample materials having chemical compositions given in Table 1 wereproduced and each examined for the tempering temperature range thatallows the yield stress to be in the range from 758 MPa to 860 MPa. Thesample materials were also examined for toughness and corrosionresistance.

TABLE 1 Chemical Compositions (unit: % by mass, the balance is Fe andimpurities) No. C Si Mn P S Cu Cr Ni Mo sol. AL N Inventive 1 0.006 0.140.46 0.012 0.0008 0.02 11.79 6.79 2.91 0.020 0.0053 Steel 2 0.006 0.140.47 0.012 0.0008 0.02 11.75 6.78 2.90 0.027 0.0030 3 0.007 0.17 0.400.014 0.0011 0.02 11.88 6.90 2.95 0.029 0.0075 4 0.006 0.15 0.47 0.0120.0008 0.26 11.79 6.72 2.91 0.025 0.0038 5 0.006 0.20 0.46 0.011 0.00100.03 11.88 7.01 3.12 0.035 0.0053 6 0.007 0.16 0.47 0.012 0.0009 0.9311.78 6.37 2.90 0.025 0.0037 7 0.008 0.14 0.30 0.017 0.0010 0 11.00 7.904.80 0.035 0.0060 8 0.004 0.15 0.45 0.011 0.0010 0 11.90 7.98 3.80 0.0250.0099 9 0.006 0.16 0.46 0.012 0.0008 0.03 11.90 6.79 2.91 0.020 0.050110 0.006 0.14 0.46 0.012 0.0010 0.02 11.70 6.56 2.91 0.020 0.0450 110.007 0.14 0.35 0.012 0.0008 0 11.91 6.81 2.94 0.020 0.0065 Comparative12 0.008 0.14 0.46 0.013 0.0010 0.02 11.84 7.21 2.91 0.025 0.0061 Steel13 0.007 0.20 0.45 0.010 0.0010 0.02 11.96 6.93 2.89 0.028 0.0051 140.016 0.17 0.71 0.015 0.0010 0.03 11.80 7.20 2.92 0.030 0.0055 15 0.0130.22 0.44 0.011 0.0010 0.02 12.55 6.12 3.08 0.026 0.0050 16 0.013 0.150.71 0.012 0.0009 0.26 11.83 6.54 2.92 0.022 0.0053 ChemicalCompositions (unit: % by mass, the balance is Fe and impurities) No. NbV Ti Zr Ca Mg La Ce F1 F2 Inventive 1 0.002 0.11 0.075 0 0 0 0 0 628.70.20 Steel 2 0.002 0.06 0.081 0 0 0 0 0 617.0 0.25 3 0.002 0.05 0 00.0007 0 0 0 626.4 0.14 4 0.002 0.06 0.079 0 0 0 0 0 626.0 0.24 5 0 0.060.093 0 0 0 0 0 602.1 0.03 6 0.002 0.06 0.079 0.0003 0 0 0 0 652.9 0.267 0 0.04 0.100 0 0 0.0006 0 0 713.5 0.18 8 0 0.05 0.088 0 0 0 0 0 604.90.30 9 0.002 0.11 0.075 0 0 0 0.0009 0 611.9 0.06 10 0 0.04 0.082 0 0 00 0.0008 631.4 0.06 11 0 0 0 0 0 0 0 0 644.0 0.04 Comparative 12 0.0020.06 0.084 0 0.0006 0 0 0 587.2 0.63 Steel 13 0 0.06 0.089 0 0 0 0 0595.3 0.12 14 0.004 0.06 0.001 0 0 0 0 0 580.3 0.98 15 0 0.06 0.088 0 00 0 0 683.3 −1.32 16 0.002 0.06 0.079 0 0 0 0 0 629.7 0.34 F1 = 922.6 −554.5C − 50.9Mn + 2944.8P + 1.056Cr − 81.1Ni + 95.8Mo − 125.1Ti −1584.9Al − 376.1N F2 = 30C + 0.5Mn + Ni + 0.5Cu − 1.5Si − Cr − Mo + 7.9

Steel having the chemical compositions given in Table 1 were melted. Asshown in Table 1, the chemical compositions of the sample materials 1 to11 were within the range of the chemical compositions according to theinvention.

The left sides of Expressions (1) and (2) are represented as F1 and F2,respectively, and F1 and F2 were obtained for each of the samplematerials. For the sample materials without Ti, “0” is entered in thebox for “Ti” in F1 and for those without Cu, “0” is entered in the boxfor “Cu” in F2.

As for all the sample materials 1 to 11, F1 and F2 were both within therange according to the invention. More specifically, F1 was 600 or moreand F2 was zero or more.

As for the sample materials 12 and 13, while the chemical compositionswere within the range according to the invention, F1 was less than 600.As for the sample materials 14 to 16, the C content exceeded the upperlimit according to the invention. Furthermore, as for the samplematerial 14, F1 was less than 600, and as for the sample material 15, F2was less than zero.

The molten steel for the sample materials 1 to 16 were cast intocontinuous casting materials. The produced continuous casting materialswere subjected to hot forging and hot rolling and made into a pluralityof steel plates each having a thickness of 15 mm, a width of 120 mm, anda length of 1000 mm. The steel plates after the hot forging and hotrolling were cooled by air to room temperatures. Using the obtainedsteel plates, the following tests were conducted.

1. Tempering Temperature Range

To start with, the obtained plurality of steel plates were quenched. Atthe time, the quenching temperature was 910° C. Then, the quenched steelplates were subjected to tempering. At the time, the temperingtemperature was varied within the temperature range from 450° C. to 650°C. The steel plates after the tempering at various temperatures weresubjected to tensile tests. More specifically, a round-bar test piecehaving a diameter of 6.35 mm and a length of 25.4 mm for the parallelpart was produced from each of the steel plates. Using the producedround-bar test pieces, tensile tests were conducted at room temperaturesbased on JIS Z2241 and the yield stresses were obtained. After thetensile tests, the tempering temperature range ΔT in which the yieldstress was in the range from 758 MPa to 860 MPa was obtained for each ofthe sample materials. Note that the 0.2% proof stress was set as theyield stress.

The tempering temperature ranges of the sample materials that allow theyield stress to be in the range from 758 MPa to 860 MPa are given inTable 2.

TABLE 2 No. ΔT(° C.) Inventive 1 110 Steel 2 80 3 100 4 110 5 45 6 80 750 8 55 9 40 10 110 11 100 Comparative 12 10 Steel 13 10 14 20 15 25 1620

In Table 2, ΔT represents the difference between the maximum temperatureand the minimum temperature among the tempering temperatures at whichthe yield stresses of the sample materials are from 758 MPa to 860 MPa.The unit is “° C.”

As shown in Table 2, ΔT was 40° C. or more for each of the samplematerials 1 to 11. Meanwhile, ΔT was less than 40° C. for the samplematerials 12 and 13 because F1 was less than 600 for them. The samplematerial 14 had a high C content, F1 was less than 600, and therefore ΔTwas less than 40° C. The sample materials 15 and 16 each had a high Ccontent, and therefore ΔT was less than 40° C.

FIG. 2 shows the relation between the tempering temperature and theyield stress in the sample materials 1 and 14. As shown in FIG. 2, thegradient of the tempering temperature curve C1 of the sample material 1whose F1 was 600 or more was small in the yield stress range of 758 MPato 860 MPa and the tempering temperature range ΔT1 was 110° C.Meanwhile, for the sample material 14 whose F1 was less than 600, thegradient of the tempering temperature curve C2 was large in the yieldstress range from 758 MPa to 860 MPa, and the tempering temperaturerange ΔT2 was as small as 20° C.

2. Toughness

The toughness values of the sample materials are given in Table 3.

TABLE 3 yield stress absorbed energy No. F2 (MPa) at −40° C. (J)Inventive 1 0.20 804.0 185 Steel 2 0.25 782.0 182 3 0.14 812.0 176 40.24 805.0 185 5 0.03 813.0 188 6 0.26 838.0 187 7 0.18 854.0 192 8 0.30808.0 190 9 0.06 841.0 188 10 0.06 807.0 190 11 0.04 773.0 194Comparative 12 0.63 848.0 160 Steel 13 0.12 815.0 165 14 0.98 851.0 16715 −1.32 844.0 81 16 0.34 841.0 171

The toughness tests were conducted as follows. The obtained steel plateswere quenched at 910° C. and tempered so that the yield stresses becomevalues given in Table 3. From each of the tempered steel plates, aV-notch test piece as wide as 10 mm according to JISZ2202 was produced.

The produced V-notch test pieces were subjected to Charpy impact testsaccording to JISZ2242 at −40° C. and examined for absorbed energy.

The unit of the absorbed energy in Table 3 is J. Since the F2 values ofthe sample materials 1 to 11 are all at least zero, the values of theabsorbed energy exceeded 100 J, in other words, high toughness resulted.Meanwhile, the F2 value of the sample material 15 was less than zero,and therefore the absorbed energy was low.

3. Corrosion Resistance

Corrosion resistance in a wet carbon dioxide gas environment wasevaluated by conducting carbon dioxide gas corrosion tests. A test piecehaving a width of 20 mm, a thickness of 3 mm, and a length of 50 mm wascut from each of steel plates quenched and tempered in the sameconditions as the above toughness evaluation. The surface of each testpiece was polished with No. 600 emery paper, then degreased, and dried.

The produced test pieces were immersed for 720 hours in a 25% NaClaqueous solution in which CO₂ gas at 9.73 atm and H₂S at 0.014 atm weresaturated. Note that the aqueous solution was kept at 165° C. during thetests.

After the tests, the test pieces were each examined for quantity losscaused by corrosion. More specifically, the corrosion loss was obtainedas a value produced by subtracting the weight of a test piece after thetest from the weight of the test piece before the test. Thepresence/absence of local corrosion at the surfaces of the test pieceswas visually inspected. It was determined that the piece had highcorrosion resistance in a wet carbon dioxide gas environment if thecorrosion loss was less than 7.7 g and there was no local corrosionfound.

The following SSC tests were conducted to examine SSC resistance in awet hydrogen sulfide environment. Tensile test pieces each having adiameter of 6.3 mm and a length of 25.4 mm for the parallel part werecut from steel plates quenched and tempered in the same conditions asthe above toughness evaluation. The produced tensile test pieces weresubjected to proof ring tests based on NACE TM0177-96 Method A. At thetime, the test pieces were immersed for 720 hours in a 20% NaCl aqueoussolution in which H₂S(CO₂ bal.) at 0.03 atm was saturated. The pH of theNaCl aqueous solution was 4.5 and the temperature of the aqueoussolution was kept at 25° C. After the tests, the presence/absence ofcrackings was visually inspected.

The result of the corrosion resistance tests is given in Table 4.

TABLE 4 carbon dioxide gas corrosion SSC No. tests tests 1 ◯ ◯ 2 ◯ ◯ 3 ◯◯ 4 ◯ ◯ 5 ◯ ◯ 6 ◯ ◯ 7 ◯ ◯ 8 ◯ ◯ 9 ◯ ◯ 10 ◯ ◯ 11 ◯ ◯

In Table 4, “◯” in the carbon dioxide gas corrosion tests indicates thatthe corrosion loss was less than 7.7 g and there was no local corrosion.In the SSC corrosion tests, “◯” indicates that there was no crackinggenerated. The sample materials 1 to 11 each had high corrosionresistance.

Although the embodiment of the present invention has been described andillustrated in detail, it is clearly understood that the same is by wayof illustration and example only and is not to be taken by way oflimitation. The invention may be embodied in various modified forms asrequired without departing from the spirit and scope of the invention.

INDUSTRIAL APPLICABILITY

The martensitic stainless steel according to the invention is applicableas a steel material for use in a corrosive environment includingcorrosive substances such as hydrogen sulfide, carbon dioxide gas, andchloride ions. The steel is particularly applicable to steel materialsfor production facilities, geothermal power generation facilities, andcarbon dioxide gas removing facilities, and steel pipes used as oilcountry tubular goods in a wet hydrogen sulfide environment and a wetcarbon dioxide gas environment such as oil wells or gas wells.

1. Martensitic stainless steel, comprising, by mass, 0.001% to 0.01% C,at most 0.5% Si, 0.1% to 3.0% Mn, at most 0.04% P, at most 0.01% S, 10%to 15% Cr, 4% to 8% Ni, 2.8% to 5.0% Mo, 0.001% to 0.10% Al, at most0.07% N, 0% to 0.25% Ti, 0% to 0.25% V, 0% to 0.25% Nb, 0% to 0.25% Zr,0% to 1.0% Cu, 0% to 0.005% Ca, 0% to 0.005% Mg, 0% to 0.005% La, and 0%to 0.005% Ce, the balance being Fe and impurities, said steel satisfyingExpressions (1) and (2) and having a yield stress in the range from 758MPa to 860 MPa.922.6−554.5C−50.9Mn+2944.8P+1.056Cr−81.1Ni+95.8Mo−125.1Ti−1584.9Al−376.1N≧600  (1)30C+0.5Mn+Ni+0.5Cu−1.5Si−Cr−Mo+7.9≧0  (2) where the contents of theelements in percentage by mass are substituted for the charactersrepresenting the elements.
 2. The martensitic stainless steel accordingto claim 1, further comprising at least one of 0.005% to 0.25% Ti,0.005% to 0.25% V, 0.005% to 0.25% Nb, and 0.005% to 0.25% Zr.
 3. Themartensitic stainless steel according to claim 1, further comprising0.05% to 1.0% Cu.
 4. The martensitic stainless steel according to claim2, further comprising 0.05% to 1.0% Cu.
 5. The martensitic stainlesssteel according to claim 1, further comprising at least one of 0.0002%to 0.005% Ca, 0.0002% to 0.005% Mg, 0.0002% to 0.005% La, and 0.0002% to0.005% Ce.
 6. The martensitic stainless steel according to claim 2,further comprising at least one of 0.0002% to 0.005% Ca, 0.0002% to0.005% Mg, 0.0002% to 0.005% La, and 0.0002% to 0.005% Ce.
 7. Themartensitic stainless steel according to claim 3, further comprising atleast one of 0.0002% to 0.005% Ca, 0.0002% to 0.005% Mg, 0.0002% to0.005% La, and 0.0002% to 0.005% Ce.
 8. The martensitic stainless steelaccording to claim 4, further comprising at least one of 0.0002% to0.005% Ca, 0.0002% to 0.005% Mg, 0.0002% to 0.005% La, and 0.0002% to0.005% Ce.